The present invention pertains to a system for detecting the position of one part relative to another without any contact between the two parts. For example, one of the parts can be a xe2x80x9ctarget devicexe2x80x9d movable relative to a xe2x80x9csensing device,xe2x80x9d and the two parts can be incorporated in or mounted on adjacent, relatively movable components of a mechanical system.
In the aircraft industry, which is one industry with which the present invention is concerned, there are a variety of situations in which it is desired or essential to know the position of one mechanical component relative to another. xe2x80x9cPositionxe2x80x9d can include the location of one part relative to another (X, Y, and Z coordinates in a Cartesian coordinate system, for example), and/or the attitude of one part relative to another (degree of relative pitch, yaw, and/or roll). For example, for optimum performance it may be necessary to know the degree of extension or retraction of an actuator, which may correspond to the position of a flight control surface such as a leading edge or trailing edge flap, aileron, rudder, or horizontal stabilizer, or the orientation of a nose wheel, or any of a variety of other components. Depending on the actuator or component, different position sensing or indicating systems have been used. For example, if the actuator is of the screw type and run by a motor, electro-mechanical counters may be used to estimate position by the number of turns that the screw type actuator has been driven. In another case, inductive sensors have been used, primarily linear variable differential transformers (LVDTs), where the change in coupling between primary and secondary coils is measured to indicate the position of a magnetically permeable rod run through a core. The ends of the LVDT can be connected to devices whose relative location it is desired to measure. Other known location sensing examples include electronic calipers and optical encoders.
In general, known sensing systems require very close and controlled spacing between the components whose relative positions are being measured. These are also subject to inherent accuracy tolerances which, ideally, should be reduced, and/or have other disadvantages such as temperature or pressure variations, or sensitivity to environmental conditions such as dust, wind, grit, electronic noise, and so on. Using an LVDT as an example, an LVDT system can be sensitive to temperature variations, must move in a linear fashion, require precise alignment, is sensitive to cable length, usually requires protection from dirt or ice, and can be sensitive to noisy electronic environments.
In contrast to true position sensing systems, there are known proximity sensors that will indicate the close presence of one object relative to another. LVDTs have been used for proximity sensing applications, as well as Hall effect sensors of the type that provide an output as a function of magnetic field strength. In the absence of structure interfering with the presence or detection of a magnetic field, a Hall effect sensor may be used to detect the fact that a component having a magnetic target has been moved close to a component on which a Hall effect sensor has been mounted. Examples of proximity sensing systems are described in U.S. Pat. Nos. 5,285,154 and 5,351,004. These patents also describe embodiments for sensing distance within a limited range.
The present invention provides a highly accurate position (location and/or attitude) sensing system which can be used for noncontacting objects or parts that are moved relative to each other, and which is operable in environments hazardous to other sensing systems, is very insensitive to temperature variations or noisy environments, can move in nonlinear, curved, or twisting directions, and needs no protection against dirt or ice.
In one aspect of the present invention, a target device is carried by or mounted on one of the parts. This target includes one or more magnets creating a magnetic field. The present invention also includes a sensing device carried by or mounted on the other of the parts, which contains an array of Hall effect sensors for detecting the magnetic field. For example, in a one dimensional or xe2x80x9clinearxe2x80x9d sensing system, the Hall effect sensors can be arranged in one or more rows extending lengthwise in the sensing direction, such direction being the direction along which the target device is moved relative to the sensing device. In such a system, the individual outputs of the Hall effect sensors are electronically scanned and processed to determine the location of the target relative to the sensing device. This can involve grouping the sensors and providing their outputs to a series of multiplexers which, in turn, are connected to a processor for analyzing the outputs and computing the location of the target.
More specifically, the processing of the individual Hall effect sensor outputs can include determining which Hall effect sensor has the highest output, corresponding to the location of the greatest magnetic field strength, and limiting the processing to the output of that Hall effect sensor and a predetermined number of sensor(s) at each side. The analog outputs of the selected sensors are converted to digital information that is evaluated mathematically to determine the target location. Depending on the requirements of a particular application, different error correction algorithms may be used for a desired degree of accuracy or failure mode correction.
Other methods are also used to determine the correct set of Hall effect sensors to evaluate for position. One such method is correlation. In this method, a vector of values corresponding to the desired signal is mathematically correlated against the vector signal set from scanned Hall effect sensors. A peak in the correlation signal indicates the center of the desired set to evaluate. This method works well when dealing with possible failed Hall elements.
Additionally, the detection system need not be the peak signal and adjacent Hall devices, but instead or in addition, one might desire to look at the zero crossing signal which results from using combinations of north and south magnets.
In another aspect of the invention, the system can be adapted for measuring or tolerating variations in the location of the target perpendicular to the normal sensing direction. In such a system the normal sensing direction can be referred to as the X direction, a direction transverse to the sensing direction but parallel to the sensing device can be referred to as the Y direction, and the direction perpendicular to the sensing device can be referred to as the Z direction. The system of the present invention is adaptable to large tolerances in the Y and Z directions while still providing accurate X direction measurements, such that precise alignment of the parts is not always required. The X direction travel need not be straight and flat, but can follow arcs, curves or twisting surfaces.
In still another aspect of the present invention, the system is adapted for measuring the location of the target in two dimensions (such as X and Y) and/or three dimensions (X, Y and Z), or any other coordinate system. In addition, a system in accordance with the present invention can be used to measure relative attitude (pitch, yaw, and/or roll positions).
Error correction can be used to compensate for manufacturing tolerances, such as unavoidable impreciseness in the positioning of the Hall effect sensors"" physical locations and unavoidable tolerances in the outputs of the Hall effect sensors themselves, which include variable gains and voltage offsets.
In another aspect of the invention, the system can be adapted for detecting the rotational angle, the speed of rotation, or the acceleration, of a swinging or rotatable component.
Because of the ability of this system to look through nonmagnetic materials, one of this system""s major attributes is that both the sensor and the target can be sealed, and thus function in hazardous environments and/or harsh environments. Detection areas such as chemical or fuel cells, or high pressure or high temperature environments can be accommodated.
In most instances, positional error of systems like LVDT""s is expressed in percent error of stroke length. This results in larger absolute errors for longer stroke sensors. The present invention has a very small error initially, and that error is constant regardless of stroke length. As a result, for longer stroke devices of the present invention, the percent error of stroke decreases as the sensing length increases.
The present invention uses a computational device like a micro-controller. This device and its associated electronics (power supply, I/O, analog-to-digital converter) can be either integrated with the sensor, or moved remote. Sometimes it is advantageous to move some of the electronics to a more benign environment to increase overall reliability and reduce system costs.
Another significant advantage of this invention is that the sensor can track several targets at the same time. This feature allows the invention to perform unique features such as measure special targets of lengths far greater than the sensor, average many targets for reduced error, or correct off axis rotation speed wobble on a circular sensor.